In accordance with the present invention, an acceleration sensor having a mass is movably supported outside its center of gravity, first electrodes on the mass and second electrodes being located at a distance therefrom forming a capacitive sensor in order to determine a change in position of the mass as a function of time. At least one spring element which generates a restoring force when the mass is deflected from its neutral position is provided on the side of the mass facing the capacitive sensor. These types of acceleration sensors are used in motor vehicles, for example, to activate safety devices, or in portable devices to detect an impact load, for example as the result of falling.
An acceleration sensor of this type is described in U.S. Patent Application No. 2006/0180896 A1. This acceleration sensor contains a generally cuboidal mass which is fixed to a base plate using at least one spring element. Located between the base plate and the elastically supported mass is at least one pair of electrodes, which, as a plate capacitor having a variable plate distance, allows the motion of the mass to be capacitively measured. If more than one pair of electrodes is provided, it is possible to detect not only a lifting of the mass from the base plate, but also a tilting or rotation. An acceleration measurement may thus be carried out in more than one spatial direction.
However, a disadvantage of this is that tilting of the oscillatingly supported mass at its base surface causes only a slight change in distance and therefore generates only a small detection signal. The accuracy of detecting accelerations which do not act perpendicular to the base surface of the mass is therefore low. Furthermore, this sensor cannot be installed in a molded housing without limiting the movability of the mass used for the detection. It is no longer possible to detect accelerations as soon as the gap present between the base plate and the mass is filled with housing material. For this reason, the costly, large housing illustrated in FIG. 4 of U.S. Patent Application No. 2006/0180896 A1 is used. Installation in small, transportable devices having a high component packing density is therefore difficult.
An object of the present invention is to provide an acceleration sensor having improved detection accuracy in all three spatial directions. A further object of the present invention is to provide an acceleration sensor which may be installed in a compact, easily manufacturable housing.
The object may be achieved according to an example embodiment of the present invention by use of an acceleration sensor having a mass which is movably supported outside its center of gravity, first electrodes on the mass and second electrodes located at a distance therefrom forming a capacitive sensor in order to determine a change in position of the mass as a function of time, and at least one spring element which generates a restoring force when the mass is deflected from its neutral position being provided on the side of the mass facing the capacitive sensor, the mass being obtainable by being exposed from a material layer and the mass is surrounded, at least at its side faces, by this material.
According to the present invention, it has been found that the oscillatingly supported mass, used for the acceleration measurement, may be exposed from a material layer, for example silicon. The pairs of electrodes used for the distance measurement are situated between the measuring mass and the support substrate, as likewise are the spring elements which are used for support. The mass may have any external shape, for example a cuboidal shape.
To allow high detection accuracy in multiple spatial directions, the elastically supported mass preferably has a suspension on only one base surface. A change in position in a comparatively high amplitude, produced by accelerations parallel or perpendicular to the base surface, is thus possible. A large deflection amplitude results in a large change in capacitance and high measurement accuracy.
To further increase the accuracy, in one specific example embodiment of the present invention the electrodes are structured. In other words, the electrodes are subdivided, and each partial surface may be connected to a separate electronic measuring system for capacitance measurement. When capacitive sensors for distance measurement are situated at two opposite ends of the mass, a tilting of the mass from its neutral position results in a decrease in the distance from one electrode and at the same time results in an increase in the distance from the other electrode. The accuracy of the measurement may be further increased by simultaneous measurement and comparison of the two values.
In one specific embodiment, the mass may also be suspended within an axis of symmetry but outside its center of gravity. As a result, only a single suspension using one spring element is necessary. An asymmetrical sensor response due to spring elements having designs which are not exactly identical is thereby avoided. In addition, further miniaturization is possible on account of the reduced space requirements for only one elastic support.
A micromechanical spring element for elastic support of the measuring mass may be produced according to the present invention by the mass having a cavity which is sealed by a diaphragm made of the same or different material. The mass is then mounted on the substrate using a thin web or a column which engages with the diaphragm. The diaphragm which seals the cavity thus forms a spring element, which may completely close off the cavity, or may be separated from the base surface of the mass at one or more boundary lines of the cavity. The spring constant of the suspension may be adjusted in this manner. For a particularly soft suspension which allows measurement of very small accelerations, multiple cavities having multiple diaphragms as the spring element may be situated one on top of the other. These may be provided in a line or be laterally offset. Furthermore, the spring constant may be adjusted over the thickness of the diaphragm. Thus, numerous parameters are available to one skilled in the art to optimally adapt the acceleration sensor according to the present invention to the intended purpose.